The United Nations will not hold another round of Syria peace talks in Geneva until officials on all sides agree the parameters for a political transition deal, which has an August 1 deadline, UN envoy Staffan de Mistura said on Thursday.
In the meantime, a series of "low-profile" technical meetings will be held in various cities to discuss issues ranging from the role of the Syrian army and national institutions after any peace deal, de Mistura said.
"I have informed the Security Council just a few days ago ... The time is not yet mature for the official third round of the intra-Syrian talks," de Mistura told reporters.
"Why? Because we are aware that a third round needs to be a concrete one," he said, adding that this meant steps toward a political transition to end the five-year war.
"The first of August is attainable, should be attainable and we should be aiming at that one," de Mistura said.
De Mistura also said he had heard from Russia about the release of a "substantial number" of fighters detained by the Syrian government, but he wanted to get confirmation and more details, including on whether some were political prisoners.
More than 100,000 people are believed to be languishing in government detention centres after five years of civil war. An unknown number are held by rebel and jihadi groups after being abducted.
A former staff member of the International Committee of the Red Cross (ICRC) joined de Mistura's team last month to tackle the issue of detainees, a move long sought by the opposition.
"(The release of detainees) may coincide with the holy month of Ramadan or (may be) a unilateral decision and gesture by the government to want to show an intention of addressing ... what is a huge concern and a huge problem, we are talking about thousands and thousands (of people)," de Mistura said.
The Syrian government has given its approval for UN land convoys to 19 besieged areas in June, but the proof will be concrete deliveries, de Mistura said, speaking after the weekly meeting of the humanitarian task force.
In the meantime, the option of air drops by helicopters and air lifts to besieged areas - which require Damascus's approval to use its air space - remain on the table, although land convoys are cheaper and more efficient, he said.
"Because if we get in by land we can do what can be done by 200 air drops in one simple long convoy," he said.